1. Field of the Invention
The present invention is related to the evaluation of discontinuity indications from non-destructive testing of objects to determine a relationship between indication size, stress applied to the object and remaining life of the object and, more particularly, to a method of determining initial allowable indication size, allowable stress and remaining life of a high pressure turbine rotor based upon ultrasonic and magnetic particle testing results.
2. Description of the Related Art
Many types of equipment and components undergo non-destructive testing or evaluation both during the manufacturing process for quality control and during maintenance to help make decisions regarding repair. Tests which are used in performing non-destructive evaluation (NDE) include ultrasonic testing and magnetic particle testing. Ultrasonic testing can be used on virtually any object with the proper selection of frequency and the construction and arrangement of transducers. Ultrasonic testing produces indications of change in composition or structure of an object including detection of minute holes or other discontinuities of structure in the interior of an object. In addition, if an object can be used to form an electromagnet, e.g., if constructed of a ferrous metal, similar discontinuities in structure or composition on the surface of the object which are difficult to detect optically can be revealed using magnetic particle testing. This method uses iron filings which indicate the fluctuations in a magnetic field formed by the object used as an electromagnet.
Both of these types of NDE are customarily used to test objects such as rotors for electricity generating turbines. A variety of methods of analyzing the data resulting from such testing is known for predicting the growth of cracks at the discontinuity indications. Typically, linear-elastic and elastic-plastic fracture mechanics (LEFM and EPFM) are used to evaluate rotors. However, high pressure and intermediate pressure steam turbine rotors are subjected to temperatures at which creep is a factor in the growth of cracks. The effects of creep are time dependent, i.e., the length of time a rotor is exposed to stress at elevated temperatures affects the crack growth rate due to creep. Conventional LEFM and EPFM analysis does not take into account time-dependent factors including creep. As a result, use of LEFM and EPFM alone in analyzing NDE indications for high pressure and intermediate pressure steam turbine rotors can result in significantly underestimating the rate of crack growth.